1. Technical Field
This invention relates to tunneling thermometry, and more particularly, to single contact tunneling thermometry.
2. Related Art
Although temperature measurements on the micro- and nanoscale are greatly challenging, they are vitally important in a large number of technological processes. Power dissipation on the scale of single nanoscale electronic elements may be a limiting factor in both future scaling of microelectronic elements and their operation at higher clock-frequencies and in confined environments. Micro- and nanoscale temperature measurements are the foundation of thermal microscopy, which is a highly versatile technique that can address power dissipation and local heating effects in conducting materials, junctions and interconnects, and may prospectively be used for optical and biological applications. Having a knowledge of temperature gradients across a material (derived from temperature maps), across an interconnect, or across a point-contact junction may lead to qualitative and quantitative estimates of thermal conductivity, which is a fundamentally important property for a wide variety of electronic and energy applications. Finally, nanoscale thermometry is a platform for fundamental discoveries in thermal transport and electron-phonon interactions, both of which need to be optimized for the design of efficient thermoelectric energy harvesting, electronic refrigeration and thermal management in microscale and nanoscale systems.
The most advanced optical methods for thermal measurements, relying on spectroscopy of infrared photons, may only provide a resolution on the order of 1 micrometer. Therefore, today, the absolute majority of nano- and microscale temperature measurements are carried out using point-contact techniques, broadly termed Scanning Thermal Microscopy, which rely on miniaturized thermocouples or resistive thermometers. Such measurements are typically carried out by bringing a thermal probe into a good mechanical and, crucially, a good thermal contact with a sample of interest and reading out the local temperature from the electronic response of the thermal probe (typically thermovoltage of the thermocouple, electrical resistance of a resistive thermometer such as a Wollaston probe, or local power dissipation). The probe may be raster-scanned across the surface of the heated material to map out local temperature variations. A documented resolution of such techniques is ˜50 nm.
Scanning Thermal Microscopy utilizes two electrical leads that come to a junction, and it is at this junction that the thermal-electronic signal will be generated. Although the point of mechanical contact to the surface can then be extended beyond the junction of the two wires, and further miniaturized, the mere fact of requiring two separate wires precludes efficient downsizing of the probe as a whole, or its integration with microelectronic devices, micro- and nano-electromechanical systems, and other related technologies.
An even more serious limitation of making a physical contact to measure local temperature is that, in contrast to macroscopic thermocouples, the temperature sensors in micro- and nanoscale contact thermometry may not equilibrate with the surface of interest. Any of these three scenarios may apply: (1) the thermal resistance of the point contact may be larger (for metal contacts ˜107 K/W) than that of the thermometer leads (˜105 K/W), in which case one should consider explicitly the flow of heat from the temperature sensor to the object of interest. This may be a very complicated problem to treat quantitatively because of the lack of detailed knowledge of the contact geometry and the resulting thermal boundary resistances. Typically, one may rely on a calibration of the method using objects with known temperatures, or a theoretical modeling of the problem, both of which are not only time consuming, but also prone to their own errors. Moreover, temperature equilibrium may be amplified with diminishing size of the two-terminal temperature probe. (2) The opposite may be true: the thermal resistance of the point contact may be poor such as in the case of bad physical contact. In this case, the sensor may equilibrate but the temperature reading may not be accurate, and also not sufficiently sensitive (depending on exact parameters). (3) The size of the measured object is smaller than the size of the probe (e.g. nanoparticles, lithographic patterns etc.) In this case the sensor itself will introduce too much perturbation, altering the temperature of the object and producing flawed measurements.